Lewis & Short

sollemnĭtas (sollenn-, solenn-, and solemn-), ātis, f. [sollemnis] (postclass.; cf.: sollemne, celebratio).

  1. I. A solemnity, festival, celebration of a day: dierum variae sollemnitates, Gell. 2, 24, 15: pristina (ludorum Isthmiorum), Sol. 7: condendi honoris, Aus. Grat. Act. 36: sacrorum sollemnitate prisco more completa, Amm. 23, 3, 7; Vulg. Deut. 16, 16 et saep.
  2. II. A usage, formality: juris, Dig. 26, 8, 19; 1, 7, 25.